The present exemplary embodiment relates to document processing systems. It finds particular application in conjunction with sensing and control of banding and will be described with a particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.
The image quality defect known as banding is a periodic modulation of color (some aspect of lightness, hue, and saturation) in the image on a printed medium that runs in the marking process direction. Banding generally occurs across the full width of an image, and may vary in amplitude in time and in the direction perpendicular to the marking process direction, i.e., the cross-process direction. Banding can be caused by a number of fluctuations that occur within the subsystems of a marking engine such as, for example, laser polygon Raster Output Scanner (ROS) facet-to-facet reflectance variation, intensity and spot size variation in a multibeam ROS, ROS polygon wobble, non-uniform motion of certain subsystems, gap variations between moving surfaces, and non-uniform photoreceptor wear and/or charging.
A typical approach to eliminate banding defects is to require the manufacture of parts/subsystems to meet tight tolerances which results in high costs. Alternative approaches include using active compensation schemes. In one compensation scheme, banding defects are sensed with optical sensors in the developed image on the photoreceptor. The development field is actuated according to a feedback control strategy in order to prevent the formation of the bands. In such an approach, accurate knowledge of banding frequency, amplitude and phase is critical at the time of the compensation, this knowledge being gained through sensing. Typically, the sensing is performed by sensing imaged targets at the photoreceptor or in the print media path. However, such sensing of the targets is performed in the midst of customer's jobs, thus using up extra printing cycles and reducing the overall productivity of the printing system.
There is a need for methods and apparatuses that overcome the aforementioned problems and others.